1. Field of the Invention
This invention relates to stabilizer combinations for Ca/Zn-stabilized PVC molding compounds.
The processing of rigid PVC molding compounds by injection molding, bottle blowing, calendering or the like at relatively high material temperatures requires optimal melt flow. If the end products are required to show high transparency, PVC types having low K values, for example in the range from 55 to 63, have to be used. Establishing good melt flow properties requires relatively high dosages of lubricants. The lubricants have to show optimal compatibility with the PVC compounds because only then can glassclear products, such as bottles or films, be produced.
2. Discussion of Related Art
Known lubricants of the type mentioned above are, for example, partial esters of glycerol, such as glycerol monooleate, glycerol dioleate and glycerol monostearate, fatty alcohols having chain lengths in the range from C.sub.16 to C.sub.18, phthalic acid esters of the fatty alcohols mentioned and also hydrogenated castor oil, i.e. the glycerol ester of 12-hydroxystearic acid. These and other lubricants are described, for example, in Becker/Braun, Kunststoffhandbuch 2/1 (Polyvinylchlorid), Carl Hanser Verlag, Munchen, 1986, pages 570-586. Other lubricants for PVC molding compounds described in the prior art include dihydroxyfatty acid glycerides obtained by hydrogenation of the corresponding epoxyfatty acid glycerides (EP-A 0 166 201) and ketofatty acid glycerides of which the ketofatty acid components are derived from mixtures of 9- and 10-ketostearic acid (DE-A 33 34 600). According to EP-A 0 166 201, dihydroxyfatty acid glycerides containing impurities of glycerides of C.sub.18 ketocarboxylic acid glycerides are also suitable for stabilizing Ca/Zn-stabilized PVC molding compounds. However, the PVC molding compounds obtained in this way contain only very small quantities (at most approx. 0.08% by weight) of the ketofatty acid glycerides.
Ca/Zn-stabilized PVC molding compounds are used in particular for the packaging of foods. A typical example of this application are water bottles, particularly for so-called still waters and CO.sub.2 -containing waters. Compounds of the type in question are additionally stabilized with epoxidized triglycerides, more particularly epoxidized soybean oil. Additional co-stabilizers, for example 1,3-diketones and/or derivatives of dihydropyridine, may be used because the stabilizing effect of CaZn systems is not very pronounced. Molding compounds of this type may contain diesters of ethylene glycol with saturated, straight-chain C.sub.18 fatty acids as internal lubricant. Thus, DE-A 38 12 014 describes a stabilizer combination for Ca/Zn-stabilized PVC molding compounds containing Ca salts of saturated, straight-chain C.sub.8-18 fatty acids, Zn salts of saturated, straight-chain or branched C.sub.8-18 fatty acids, epoxidized glycerides of unsaturated straight-chain C.sub.16-22 fatty acids having oxirane values of 6 to 8 and iodine values of less than 5 and the above-mentioned ethylene glycol diesters of saturated, straight-chain C.sub.16-18 fatty acids. The compatibility of the ethylene glycol diesters mentioned above with PVC molding compounds is comparable with that of hydrogenated castor oil; i.e. PVC molding compounds treated with these lubricants have substantially the same transparency properties.
Accordingly, the problem addressed by the present invention was to provide a stabilizer combination for Ca/Zn-stabilized PVC molding compounds containing an internal lubricant which would be superior in its effect to known internal lubricants, more particularly hydrogenated castor oil and the diester of ethylene glycol with saturated straight-chain C.sub.16-18 fatty acids.